Solar System
A general introduction to the neglected celestial bodies of humanity's home system in 40k.
Sol/Sun
It is the sun, yellow, G Class, with 5 billion years of life left under normal circumstances. General graveyard of heretics and their possessions (no pun intended).
Mercury
Mining world. That's it. Literally. The lore is that long. However much ore is left after tens of thousands of years is a matter of debate.
Venus
An Imperial forge world. Before the Great Crusade it was ruled by the War Witches, whoever they are, but was brought under the control of the Imperium by the early Iron Warriors.
Earth/Holy Terra
The capital planet of the Imperium, the homeworld of mankind and the most important world in the whole of the Imperium. Unless you are an astronaut, you are here.
Luna/Moon
Used to be ruled by gene-cults with a lot of growth vats before the Emperor set off uniting the Solar System. Refusing to be annexed and sending the envoy back as a somehow living sack of flesh, this caused big Emps sent the XVIth Legion(and co.) upon them, with the gene-cults finally saying "OK, we surrender, call off your wolves." As such, the Luna Wolves were christened.
Mars/Sacred Mars
Home of the Adeptus Mechanicus, and the biggest forge world of all by several degrees of magnitude. Phobos is also a Mini Forge World / Defense Platform while Deimos got moved to Saturn, specifically Titan.
Jupiter
A gas giant Forge World, though the entire population is in orbit on one of the moons or in the various stations and docks that surround it. Jupiter is THE Imperial Navy's foremost shipyard and drydock. It only makes ships, though the planet is also ringed by a dense cloud of broken ships that are waiting to be scrapped and re-purposed into new materials. It is a hotly contested Imperial political debate as to whether Jupiter is soverign to the Mechanicum or the Imperium. They went totally green though, the hippies.
It once had a Forge Moon, Ganymede, that was used for experimentation on recovered Squat technology (Warp batteries). However, the experiments went awry and the moon was declared lost forever and declared a quarantine zone. However, since Roboute Guilliman returned, the Grey Knights got off their asses and recovered the moon, allowing the Imperium to convert it into a Hub-Fortress for the supply of the Indomitus Crusade.
Saturn
Though an inhospitable gas giant, the rings and many of the moons of Saturn were populated prior to the Great Crusade and formed a pocket empire called the Saturnyn Ordo that were occupied by some unnamed Xenos race. Given the Ordo's forces were called Void Hoplites, they were likely the Greeks to Terra's Romans, like the Interex army.
The worlds were swiftly liberated by the fledgling Imperium and were absorbed fully shortly thereafter where they became one of the first recruiting worlds of the Solar Auxilia and formed a cornerstone of the Imperial Army.
At some point during the Horus Heresy, the Saturnyn Ordo was seemingly dismantled and the rings and moons of Saturn were designated a quarantine zone by Malcador. Access became heavily restricted as he forwarded his plans to create the Grey Knights under the veil of secrecy with standing order to destroy any unregistered ships that approach.
Later, the moons were ravaged by the Night Lords Legion during the Solar War as a prelude to the invasion of Terra. Though the depopulation of the planet probably aided in keeping the secrets of the Inquisition after the war ended, and the planet and its moons have remained an Inquisitorial stronghold ever since.
- Titan - Homeworld of the Grey Knights chapter. Is itself orbited by Deimos, once the moon of Mars that has since been relocated. It now acts as the Grey Knight's own private Forge World.
- Encaladus: Once upon a time this was the capital of the Saturnyn Ordo, where they signed the treaty with the Emperor and joined the Imperium. Is now the Palace-Fortress of the Ordo Malleus where many inquisitors have private estates.
- Iapetus: Formerly home to a hold-out of anti-Imperial rebels who fought against the Death Guard and lost. Is now home to a naval fortress, although they serve the Inquisition and Grey Knights almost exclusively.
- Mimas: Inquisitorial prison, holding the worst criminals kept in isolation and guarded by psychic wards, gun servitors and their own Stormtrooper regiment. The fact that the prison guards are special forces men should give an idea as to how much of a hellhole this place is.
- Tethys: Designated a Death World due to daemons trapped on the surface that are used as a training tool for aspirant Grey Knight Paladins who need to go to the surface and make a pilgrimage without wearing their armour. Canon differs on whether Tethys is also the location of the archive of the massively fortified Librarium Daemonica or whether it is located on Titan itself.
Uranus
Uranus has no solid surface and its 27 moons were too small for permanent settlement (The largest: Titania is less than half the radius of Luna). Nevertheless, it was surrounded by a series of orbital cities populated by peaceful artisans called the Azurites. They built a stable warp route called the Elysian Gate allowing ships to bypass the normal Mandeville points in the Sol System.
The Azurites peacefully agreed to become absorbed into the Imperium of Man, but were assaulted by pirates who initiated what would later be called the Unheard War. The Azurites petitioned the Imperium for aid and a force of fifty Imperial Fists were sent to provide support. The Space Marines quickly broke the pirate invaders within twenty minutes of battle, but as a parting gift, the pirate fleet fired boarding torpedoes loaded with psykers infected with a psychic plague called the Screaming, which was contagious through sound and caused its victims to mutate horribly and die screaming.
As the only vaccination against the plague, the Imperial Fist forces purposefully deafened themselves and fought through the waves of mutated citizens to the station reactor and initiated a self destruct, preventing the plague from spreading any further. For their sacrifice, the Emperor ordered the Bell of Lost Souls to ring for the first time.
Despite the small size of its moons, they were heavily fortified during the Horus Heresy and acted as Rogal Dorn's "Second Sphere" of defense during the Solar War. These forces were overwhelmed by Perturabo and a fleet of over 4,000 vessels brought through the Elysian Gate. The Imperial Fist fleet avoided engagement by sniping as many ships as they could from their fortresses and managed to hold off against the sheer number of invaders for around 36 hours before retreating, saving themselves for later engagements around Jupiter and buying more time to withdraw to Terra.
Uranus was only reclaimed during the Great Scouring.
Neptune
Neptune's moons were settled during the Age of Terra some point between M15-M18, though the population devolved into hideous mutants.
The IXth Legion were given the task of bringing them to compliance, and fought a bloody war through the tunnels of its moons that cost them 12,000 men, though due to the beneficial quirks of their gene-seed they were able to replenish those numbers directly from the conquered mutant population.
The moons of Neptune were frequently infiltrated by chaos or genestealer cults at later periods.
Pluto
The dwarf planet Pluto is the site of several relay stations that allow for early detection of incoming vessels to the solar system. It (along with Uranus) also houses the Khthonic Gate warp gate allowing ships to bypass the normal mandeville restrictions, making the location a strategic asset which is heavily fortified.
Prior to the Siege of Terra, Rogal Dorn seems to have killed Alpharius during the Battle of Pluto in the Horus Heresy, though this was simply a prelude to the proper Solar War.
A massive fleet of Sons of Horus swept in through the Khthonic Gate and successfully landed on the fortified moon of Kerberos, and managed to successfully turn its guns on the loyalist defenders. Sigismund ordered a full retreat, but he had predicted this outcome and also ordered his Tech Priests to initiate an overload which destroyed the moon outright, scattering the traitor fleet and allowing Sigismund to belay the retreat order turn and counter-attack. With few options available to the traitors, "Little" Horus Aximand assaulted Sigismund's flagship and managed to capture it, although he was bested in personal combat before Sigismund managed to escape and withdraw the remainder of his fleet to Terra, having bought the loyalists several days of time.
Irl Fluff that no one cares about
The Sun takes up nearly all of the mass (99.86% of it!) in the solar system. Occasionally shits out solar flares that create auroras and, if they're big enough, can wipe out electronics or even everything that lives. It'll also get swole as fuck and turn red in a few billion years and eat Mercury and Venus, roast Earth and wipe away Saturn's rings.
Mercury has a big-ass core when compared to the rest of the planets in the system. As it rotates, one side gets fried by the sun and the other gets frostbitten. It also has lots of pits with flat floors and bright circles around them. How'd they get there? No one knows.
Venus is the closest the solar system has to a Death World. Clouds of acid that kill whatever tries to walk around on the surface, a thick, opaque atmosphere that traps the sun's heat, ungodly amounts of volcanoes and the planet turning itself inside out every 100 million years all mix into one extreme cocktail that makes Venus a not fun place to be.
Then there's Earth, the planet you're standing on. Life just sort of popped up some 4.1 billion years ago and stuck around since. The place basically got exterminatus'd five times over the course of its history. The worst one was performed by itself. And some people think that the sixth exterminatus is happening right now thanks to us humans.
Next door is Mars, Earth's smaller cousin. It originally might have had water and life on it at some point, but because of its low gravity the breathable air fucked off and so did the water, killing whatever might have lived there. Nowadays, it has a shitty atmosphere, one giant mountain and a canyon that could stretch across the USA. Also its two moons might be captured asteroids.
In between the orbits of Mars and Jupiter is a belt of asteroids, often referred to as the Main Belt, or just the Asteroid belt, [[Fail|despite not being the largest collection of asteroids,] and consists of several smaller asteroid belts. The most notable asteroids in this belt are Ceres, a dwarf planet with some evidence of a subsurface brine revivor, and Vesta, the largest remnant of a protoplanet that has been hit by so many other asteroids that we have found parts of it on Near earth asteroids and an entire collection of meteorites from there.
Jupiter is the biggest planet orbiting the Sun. Period. So big in fact, that it can be bright enough to cast its own reflected light on Earth. Jupiter is also what amounts to a vacuum for incoming asteroids, as any that try to sneak by Jupiter get sucked up by its gravity and collide with Jupiter or sucked into two gravitational anomalies known as LaGrange points and become a member of a group of asteroids known as the trojan belt. And though it has a zillion moons, people really only care about four of the biggest and closest moons, called the Galilean moons. Io is the closest of the four, and its closeness really fucked it over because it's pretty much shitting its brains out 24/7. Europa is literally a smooth criminal and it probably has a fuckhuge ocean under its icy crust. Ganymede is not only the biggest of the four, but the biggest moon in the Solar System. And it also probably has another ocean under its surface. Calisto is the farthest of the bunch, the oldest moon of the bunch and was sentenced to death for heresy by several firing squads that shot meteors. In fact, Calisto is so covered in craters that the entire surface is just crater. Any new meteor that lands will just make new craters over the old ones.
Saturn has fuckhuge rings that actually made the first person to see the planet with primitive telescopes thought that the rings were two other planets. Though mostly featureless, the occasional storm can make bits of Saturn's surface look like the atmosphere of a daemon world. It also has a bunch of huge moons, including the only moon in the system with a visible atmosphere. And that moon is Titan, which has lakes of liquid methane. Methane has to be extremely damn cold to be a liquid, so this moon might be considered a Death World. Maybe. Iapetus has no atmosphere, but some jackass painted part of it black for no other reason than to just make the place look cool, but it just looks like it was sprayed with shit. Then there's Mimas, which has a giant crater on its surface that makes it look like the Death Star. Enceladus is in the same boat as Ganymede and Europa, except people actually know that there's legit water under the surface thanks to some machines we threw at Saturn. And the moon is pissing out water vapor right now.
Uranus has a name that results in it being the butt of quite a few jokes. It also rotates on its side for some fucking reason. And because of this it has sideways rings too.
Neptune is the bluest planet of literally every damn planet in the solar system. It also has a big moon that's probably a planet from beyond Pluto that got captured by the planet's gravity. And in 3 billion years or so, this moon will either crash into Neptune or turn into another ring system a la Saturn.
Pluto is the last of the bunch, and it has the largest moon in comparison to the planet it orbits. In fact, the moon is so damn big that it's more like they orbit around a point between each other. It also has a heart shape on its surface. It was recently demoted to the new category of Dwarf planet after it was found to be a member of a large asteroid belt, larger than the so-called Main belt, known as the Kuiper belt.